The present invention relates in general to a heating system which preferably employs a multi-stage heat pump for utilization of waste heat streams for the heating of buildings, the production of domestic hot water, and for other processes requiring heat streams under 200.degree. F. The waste heat stream which is a by-product from a power plant is a stream of usually heated water at temperatures in the vicinity of 100.degree.-120.degree. F. This invention is concerned with controls for the heating system so as to provide an optimum operating system. Although the system is discussed as being used in conjunction with a waste heat stream, it is understood that similar systems can also be designed for use with any heated fluid stream.
It is theoretically possible to use the waste heat stream directly for heating puposes. However, there are factors which make such use undesirable or impractical. Normally, to maintain carrying charges low on the transmission and distribution piping, the piping is made as small as possible. However, to do this one must make the mass flow as small as possible which results in large temperature drops across the utilization apparatus. High temperature drops result in outlet temperatures in the 70.degree.-90.degree. F. range. Temperatures this low thus require oversize heat transfer means (radiators), which makes the system too costly.
Another alternative instead of the use of oversize radiators is to employ a heat pump to raise the temperature of the heated fluid above the temperature of the heating fluid. In this way, the heat pump decouples the building heating system (heated fluid) from the waste heat stream (heating fluid). This use of a heat pump has been suggested by several authors, notably Illeri, et al (transactions of the ASME, Journal of Engineering for Power, July, 1976, p. 309 ff). In addition, the Department of Defense has done some experimentation in this area. See the following report; "Management of Power Plant Waste Heat in Cold Regions", by Haldor W. C. Aamot, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, N.H., December, 1974. This prior work proposes the use of waste heat at the normal condenser outlet temperature, and employ a heat pump based on more or less conventional water chillers.
In accordance with the present invention a new system has been devised for the utilization of waste heat including the controls to be used to make optimum use of the waste heat. The present invention employs a multi-state concept for the heat pump.
The multi-stage concept has been used in the refrigeration art (see, for example, the 1967 ASHRAE guide, p. 129). However, although series connected water chillers are occasionally used for refrigeration purposes, usually these machines are connected in series on the evaporator side, and in parallel on the condenser side. Furthermore, the refrigeration art does not teach the special control features incorporated into the design of the present invention because of the different operating conditions in refrigeration systems which do not lend themselves to the control in accordance with the present invention.
The heat pump of the present invention may be arranged either as a counter flow heat pump or a parallel flow heat pump. In the counter flow pump, the heating fluid and the heated fluid flow in opposite directions through the evaporators and condensers, respectively, while in the parallel flow pump, both streams flow in the same direction. Each stage in the counter flow arrangement has the same temperature lift, while the lifts are different for each stage in the parallel flow arrangement. Each arrangement has its own particular advantages and disadvantages, which will be discussed hereinafter. It can be shown from a detailed thermodynamic argument, not set forth in detail herein, that the coefficient of performance is independent of which arrangement is chosen.
One important feature of the present invention is the control that is used in association with the heat pump to optimize this system for waste heat applications. In particular, at low loads, the required heat fluid temperature may be less than the entering heating fluid temperature. This condition is not encountered in refrigeration applications employing water cooled water chillers because normal chilled water temperatures are of the order of 42.degree. F. This is sufficiently close to the freezing point that normal cooling towers (which are almost universally used for heat rejection) are frequently heated to preclude freeze-ups, which in turn precludes use of the condenser water for providing cooling without running the refrigeration. Consequently, the apparatus used in the refrigeration field has not been controlled at all in accordance with the control of the present invention.
With a properly designed waste heat utilization system, the heated fluid temperature may be less than the entering heating fluid temperature as much as 25-50% of the time. Therefore, in accordance with the invention an economic advantage is realized by not running the heat pump during these low load periods. Instead, a heat exchanger is used to perform heat transfer at low loads. In addition, further passive evaporator-condenser action occurs with the compressor of the heat pump being by-passed. This is especially advantageous with the counter flow arrangement.
In a normal refrigeration machine, refrigerant will flow from the evaporator to the condenser without the compressor being operated as long as the evaporator saturation temperature is greater than the condenser saturation temperature. Flow rate is limited by the pressure drop in the compressor and in the refrigerant flow control device. To permit maximum heat flow from the evaporator to the condenser, the compressor and refrigerant control devices are both by-passed in accordance with this invention whenever the evaporator temperature exceeds the condenser temperature by a certain amount. When a refrigerant control device is by-passed, the entire charge of refrigerant ends up in the evaporator so that that vessel is designed with a sufficient liquid capacity to hold the entire charge.
In accordance with the control of this invention there are at least two different modes which are dependent upon the demand (load). In the low load mode, heat transfer to the building heat system occurs without operation of the heat pump. In this mode the heat transfer occurs with the heat exchanger operating in combination with passive evaporator-condenser action. Once the load increases past a predetermined point, the heat pump is activated and the heating fluid flow is regulated to maintain a constant heating fluid outlet temperature. The control scheme of the invention is predicated on a constant mass flow of the heated fluid, and variable mass flow of the heating fluid. At short peak loads the total mass flow into the heat pump is limited thus relying upon the heat pump to extract more heat from the heating fluid to meet these peak load conditions.